The long term objectives of this proposed research project are to identify some of the situational parameters that affect subjects' hypothesis construction and testing behaviors under conditions of uncertainty. Special emphasis will be placed on identifying conditions that lead to systematic biases in evaluating evidence, including conditions predicted to result in erroneous conclusions, gross overconfidence, and tests that are predisposed toward confirming the hypothesis under consideration. To this end, the proposed research employs a number of simple reasoning tasks in which hypotheses are constructed, tested, and evaluated. For example, in one task, subjects are told that an exemplar (e.g., sparrow) belongs to a correct category (e.g., animals) that is specified on a hidden answer key. The goal is to discover the correct category by testing the membership status of additional exemplars. Through the use of such tasks, the effects of problem parameters, such as hypothesis saliency and number of competing hypotheses, on dependent variables, such as estimated hypothesis strength, test choices, and solution rates, can be accurately measured. Besides furthering our understanding of the cognitive processes underlying human reasoning, this research has tremendous importance for those social institutions that require both professionals and lay people to make reasoned assessments of empirical evidence. Subject's behaviors on experimental hypothesis testing tasks has long been recognized as being relevant to the behaviors of clinicians who must arrive at diagnoses and treatment plans under uncertain conditions. In addition, this research has obvious relevance for the medical, judicial, and educational systems, all of which involve hypothesis construction and testing skills.